In this new post to his blog Davey Shafik looks at an alternative to the traditional arrays most scripts use - something a little faster and more specific: SplFixedArray, part of the Standard PHP Library included with every release.
The SplFixedArray class provides a super-fast, fixed size array implementation. There are some limitations however, first you must use numeric keys and secondly you cannot use anonymous assignment (i.e. $array = 'value';). You’ll notice one requirement was missing, that it should have a fixed size. While having a fixed size is what will bring you the speed increase it’s actually not a requirement that the size be fixed.
Because of these restrictions, the SplFixedArray is faster than its cousin - between 20 and 40 percent faster, depending on the size of the array. He includes a few snippets in the the post - one showing how he benchmarked the differences against simple arrays and another showing a more advanced example with another SPL type, a FilterIterator.